Physio-chemical evaluation of asphalt-aggregate interactions

The durability of selected asphalt concrete components and mixtures from six projects were evaluated with the newly recommended net adsorption test and the more common ASTM D4867 (a modified Lottman), respectively. The net adsorption test was used to assess the durability of the adhesion of the asphalt to the aggregate surface in the presence of water. The ASTM D4867 method evaluated the retained strengths of compacted mixture resistance after freeze/thaw conditioning.

The net adsorption results indicated at least two of the six projects could be susceptible to moisture related adhesion problems. The test method was adjusted to use the full aggregate gradation rather than only the fine fraction. A comparison of these results to those originally reported by Strategic Highway Research Program (SHRP) researchers showed good agreement in both trends of results and withinlaboratory test method variability (about 0.14 mg/g).

The mixture testing indicated that three of the six projects had tensile strength ratios of less than 70 percent and could be expected to show some evidence of moisture related damage. An evaluation of these results suggested that any moisture sensitivity could be due to mix design parameters such as lower film thicknesses, and lower initial strengths rather than a chemical loss of adhesion at the asphalt-aggregate interface. The influence of conventional mix design parameters on the moisture sensitivity of asphalt concrete mixtures should be more thoroughly investigated prior to any consideration of the use of additives.